Gas turbines for power generation systems must satisfy the highest demands with respect to reliability, power, efficiency, economy, and operating service life. Modern high-efficiency combustion turbines have firing temperatures that exceed about 2,300° F. (1,260° C.), and firing temperatures continue to increase as demand for more efficient engines continues. Many components that form the combustor and “hot gas path” turbine sections are directly exposed to aggressive hot combustion gases. The use of coatings on turbine components such as combustors, combustion liners, combustion transition pieces, combustion hardware, blades (buckets), vanes (nozzles) and shrouds is important in commercial gas turbine engines.
Coatings, such as thermal barrier coating systems, contribute to desirable performance characteristics and operating capabilities at elevated temperatures. Typical thermal barrier coating systems include a bond coat disposed on the substrate of the turbine component, and a thermally insulating top coating, referred to as the “thermal barrier coating,” disposed on the bond coating. The bond coat provides oxidation and hot corrosion protection to the underlying substrate of the turbine component. Bond coat may also provide an interface for the thermal barrier coating to adhere.
U.S. Pat. No. 5,073,433 (the “'433 patent”) discloses a thermal barrier coating having a density greater than 88% of the theoretical density with a plurality of vertical cracks homogeneously dispersed throughout the coating to improve its thermal fatigue resistance. The '433 patent discloses that if the density of the coating is less than 88% of the theoretical density, the formation of the cracks is effectively prevented. Hence, this coating type has become known as a dense, vertically-cracked (“DVT”) thermal barrier coating. The '433 patent discloses forming the DVT thermal barrier coating by forming a monolayer having at least two superimposed splats having different temperatures, and cooling the monolayer and repeating the process to form the DVT thermal barrier coating.
However, the DVT thermal barrier coating of the '433 patent is difficult to produce, has a density which may be undesirable for certain applications, and may be subject to spallation under certain operating conditions. In addition, the process of the '433 patent does not permit formation of tailored regions of vertical cracking, and does not permit welding operations on the outer diameter of a combustion liner while the thermal barrier coating remains on the inner diameter of the combustion liner.